Stewart, who has spent his entire nine-year NFL career with Carolina, had a team-high 824 yards and nine touchdowns on 218 carries last season.
General manager Dave Gettleman said the Panthers extended Stewart at an age when the production of many running backs declines because "he can still flat out play.''
"A couple of weeks ago I sat back and watched every one of his snaps,'' he said. "He's still playing at a very high level. You guys know my offseason philosophy. It's really about setting up your club so that in the draft you can take the proverbial best player.
"I really feel like we've done that.''
Gettleman didn't rule out the Panthers taking a back with the No. 8 overall pick, but securing Stewart had nothing to do with that or giving the team cap relief.
Stewart was set to count $8.25 million against the cap before the extension.
"He's like a fine wine right now,'' Gettleman said of Stewart. "It has nothing to do with cap relief. Jonathan is still playing at a very high level."
"It means a lot. This is a special place," Stewart said Friday. "... I'm not satisfied by any means. Last year was disappointing. We just have to get back to the mindset of winning, and it's really about the little things."
Earlier this offseason, Panthers coach Ron Rivera specifically mentioned Stewart as a key to helping take pressure off quarterback Cam Newton.
"The biggest thing, our realization is that our primary runner cannot be our quarterback," Rivera said during an appearance on ESPN's NFL Insiders in February. "We're going to look to do a little more backfield sets in terms of two backs and 12 personnel, with the extra tight end as the lead blocker.
"We've got to get guys like Jonathan Stewart rolling. We've got to put it in our backs' hands and get the ball to our playmakers."
Stewart agrees with Rivera's thinking.
"The main thing we have to do better is protect Cam, whether that's pass-blocking or running the ball to give him some relief," Stewart said Friday. "That's going to mean the world for us. It's a quarterback-driven league, and we have the best quarterback in the league. It starts right there, and that's what we've been doing this offseason. I think we're headed in the right direction."
Stewart ranks second in franchise history with 6,638 rushing yards -- averaging 4.42 yards per carry -- and has 51 total touchdowns.
As for speculation that the Panthers will select another running back in the draft, Stewart embraces the competition.
"It's a good thing. You always want fresh legs," Stewart said. "Fresh legs mean a lot, especially in the fourth quarter. Having somebody potentially come in here ... there are a lot of good running backs in this draft class, a lot of talent. Definitely open arms to get somebody in here that wants to win and understands that. We're better as a fist than we are as an open hand."
ESPN's David Newton contributed to this report.